The present invention relates to electrostatic imaging apparatus, and more particularly to apparatus for transferring and fusing toned electrostatic images to plain paper and the like.
Electrostatic imaging processes initially require the formation of a latent electrostatic image, typically by electrographic or electrophotographic means, on a dielectric member. The latent electrostatic image is subsequently toned, and the toned image may be transferred to a further member. In the usual case, the toned image is transferred to plain paper. The transferred image is usually subsequently fused or fixed to the receptor medium, to provide enhanced durability and permanence.
Various toner image transfer methods are known in the prior art. The transfer may be accomplished electrostatically, by means of a charge of opposite polarity to the charge on the toner particles, the former charge being used to draw the toner particles off the dielectric member and onto the image receptor. Patent illustrative of this transfer method include U.S. Pat. Nos. 2,944,147; 3,023,731; and 3,715,762. Alternatively, the image receptor medium may be passed between the tonerbearing member and a transfer member, and the toner image transferred by means of pressure at the point of contact. Patents illustrative of this method include U.S. Pat. Nos. 3,701,966; 3,907,560; and 3,937,571. Usually, the toner image is fused to the image receptor subsequently to transfer of the image, at a further process station. Postfusing may be accomplished by pressure, as in U.S. Pat. No. 3,874,894, or by exposure of the toner particles to heat, as in U.S. Pat. No. 3,023,731 and Re. No. 28,693.
It is possible, however, to accomplish transfer and fusing of the image simultaneously, as shown for example in the patents cited above as illustrative of pressure transfer. This may be accomplished by a heated roller, as in Re. No. 28,693, or simply by means of high pressure between the image-bearing dielectric member and a transfer member, between which the image receptor passes. Apparatus utilizing high pressure for simultaneous transfer/fusing (termed "transfixing" by the inventor) is disclosed in U.S. Pat. Nos. 4,195,927 and 4,267,556; both are commonly assigned with the present invention, which is of the nature of an improvement thereto.
A problem which is typically encountered in transferring a toner image solely by means of pressure is the existence of a residual toner image on the dielectric member after image transfer, due to inefficiencies in toner transfer. These residual toner particles require scraper blades or other removal means, and can eventually accumulate at the various process stations associated with the dielectric member, including the apparatus for forming the latent electrostatic image. These toner accumulations decrease the reliability of the apparatus, necessitating service at intervals. Furthermore, such inefficiencies in toner transfer may lead to mottling of the images formed on the image receptor sheets.
French Patent No. 2,075,798 discloses apparatus for transferring a toner image onto an image receptor, involving an imaging roller and a pressure roller arranged to be normally in contact under high pressure. Various embodiments are disclosed for achieving a slippage between the surface of the imaging roller (from which the toner image is transferred) and the image receptor. These include covering the pressure roller with an elastic material, and driving the rollers to achieve a peripheral speed differential. The apparatus is said to achieve lower transfer pressure requirements and provide higher toner transfer efficiency. There is no suggestion of skewing the rollers to achieve these advantages, however.
U.S. Pat. Nos. 3,990,391; 4,188,109; 4,192,229; and 4,200,389 are representative of the prior art of pressure fusing apparatus employing skewed rollers. The main advantage and primary objective of skewing such pressure fusing rollers is to provide a uniform pressure distribution along the nip. This is said to reduce the loading requirements, while providing improved fusing quality across the toner-bearing sheet. These patents are not concerned with toner transfixing, and in particular with the property of toner transfer efficiency. There is no concern with providing a surface motion differential at the roller-sheet interface, and hence with achieving sufficient angular skew for this purpose. In fact, U.S. Pat. No. 3,990,391 at col. 2, lines 1-8, teaches the desirability of minimizing angular skew in order to reduce the wrinkling of the toner-bearing sheet.
Accordingly, it is an object of the invention to provide improved electrostatic imaging apparatus for pressure transfer of a toner image from a dielectric surface to a receptor sheet. A related object of the invention is to provide a transfer method which effects simultaneous fusing of the toner image.
It is a paramount object of the invention that the simultaneous pressure transfer/fusing be characterized by a high efficiency of toner transfer.
It is a further object of the invention to minimize the amount of residual toner on the dielectric image member after transfer/fusing. Related objects are increased reliability of electrostatic imaging apparatus, and decreased service requirements.
Yet another object of the invention is the enhancement of image texture. A related object is the avoidance of mottling of the transferred toner image.
A further, related object of the invention is to provide apparatus which allows the creation of high quality toned images at high speeds.